FIGS. 8A to 8E are schematic illustrations showing a mechanism of electric discharge machining. In the drawing, reference numeral 1 is an electrode, reference numeral 2 is a workpiece, reference numeral 3 is an arc column, reference numeral 4 is a working solution, and reference numeral 5 represents chips produced in the process of electric discharge machining. The workpiece 2 is being machined by electric discharge machining through the cycle from the following items (a) to (e) which correspond to FIG. 8A to FIG. 8E. Each item is described as follows. Item (a) is a process in which the arc column 3 is formed by the generation of electric discharge, item (b) is a process in which the workpiece 2 is locally melted and the working solution 4 is vaporized by thermal energy generated by electric discharge, item (c) is a process in which the working solution 4 is vaporized and an explosive force is generated by the vaporization of the working solution 4, item (d) is a process in which a melted portion (chips 5 generated in the process of machining) of the workpiece 2 is scattered, and item (e) is a process in which the workpiece is cooled by the working solution 4 and solidified so that insulation between the electrodes are recovered. When this cycle is repeated highly frequently, the workpiece 2 can be machined.
The above electric discharge machining acquires a firm position as a technique of machining metallic dies and is put into practical use in the fields of automobile industry, electric appliance industry, semiconductor industry and so forth. However, since electric discharge machining is a method of machining in which the phenomenon of electric discharge is utilized as shown by the cycle illustrated in FIGS. 8A to 8E, it is common that electric discharge machining is applied when the workpiece 2 made of conductive material such as material of iron is machined.
Concerning the method of conducting electric discharge machining on the workpiece 2 made of insulating material, there is provided a method disclosed in the official gazette of the Japanese Unexamined Patent Publication No. Sho63-150109. According to the method, a conductive film is coated onto a surface of insulating material by means of flame coating or vapor deposition, and then electric discharge is conducted on insulating material in a working solution containing carbon. For the object of abolishing a specific processing device for applying the above conductive film onto the surface of insulating material, the Japanese Unexamined Patent Publication Nos. Hei7-136849 and Hei9-253935 disclose a technique in which a workpiece made of insulating material or high resistance material is made to directly come into contact with conductive material and subjected to electric discharge machining in a working solution containing carbon.
FIG. 9 is a schematic illustration showing a composition of a wire electric discharge machine disclosed in the Japanese Unexamined Patent Publication No. Hei9-253935. In the drawing, reference numeral 1a is a wire electrode, reference numeral 2a is a workpiece made of insulating material or high resistance material, reference numeral 4a is a working solution containing carbon, reference numeral 6 is a table on which the workpiece 2a is put, reference numeral 7 is a fixing jig, reference numerals 8a, 8b are working solution nozzles, reference numerals 9a, 9b are wire guides, reference numeral 10 is a feeder piece, reference numeral 11 is a working solution supplying pump which is a member for supplying a working solution, reference numeral 12 is conductive material, and reference numeral 13 is a machining electric power supplying member. The workpiece 2a and conductive material 12 are fixed being joined to each other by the fixing jig 7. The wire electrode 1a is positioned at a joining portion of the workpiece 2a and the conductive material 12 by a positioning member not shown in the drawing. When machining electric power is supplied between the wire electrode 1a and the workpiece 2a and also between the wire electrode 1a and conductive material 12 by the machining electric power supply member 13, first, electric discharge is generated only in a portion of the conductive material 12, and then machining is conducted by thermal energy and a component of the electrode is transferred to the workpiece in a portion of the workpiece 2a closest to the conductive material 12. After that, electric discharge is also generated in the transferred portion of the workpiece 2a. In this way, machining of the workpiece 2a proceeds by a shock of electric discharge and thermal energy generated by electric discharge. According to the progress of electric discharge machining, the component of the electrode is transferred onto the surface of the workpiece 2a to be machined, and the working solution 4a is thermally decomposed. Therefore, carbon contained in the working solution 4a attaches to the workpiece 2a in the form of crystalline carbon, the electric resistance of which is relatively low, so that a conductive film can be formed. Electric discharge is generated on the conductive film formed in this way, and the workpiece 2a is machined.
As described above, the principle of the conventional electric discharge machining technique in which a workpiece made of insulating material or high resistance material is machined by electric discharge is that the workpiece is machined through a conductive film formed on the workpiece.
However, when the above conventional electric discharge machining technique is adopted, machining can not be stably conducted and further quality of a machined surface is not high. For the above reasons, the above conventional electric discharge technique has not been put into practical use on a full scale.